Abstract

In order to compare the mid-ir flux of brown dwarfs (BD’s) to the predictions of current atmospheric models, we observed four BD’s in multiple systems with the VLT/VISIR in three narrow band filters. In general the measurements were in good agreement with the predictions. Only for HD130948BC discrepancies between two observations at 11.5mm are notable, suggesting that the object might be variable. Thus we re-observed the BD, monitoring it over three half nights, proving that the object is not variable. But the mid-infrared also offers possibilities to search for brown dwarf or planetary companions to near-by, bright stars. The favourable flux contrast and the overall better PSF shape at longer wavelengths, enables us to detect companions at separations of only 1” to 3”. Thus for thirteen stars we conducted observations with T-ReCS and VISIR, reaching sensitivities of 3mJy (3sigma) at 2”. Using the polarimetric differential imaging (PDI) mode of VLT/NACO we observed twelve near-infrared excess stars. Thereby the PDI technique allows us to trace the scattered (i.e. polarised) light from the circumstellar disc very close to the central star. Here we analyse the polarised intensity distribution and characterise the polarised vector pattern, exhibited by the targets.